Glass fiber reinforced plastics have been used widely in the automotive industry in recent years. These plastics include sheet molding compound, glass fiber reinforced reaction injection moldable materials and various other glass fiber reinforced plastics. The advantages of lightweight, high strength, rust proofing and relatively low cost make these materials ideal for many automotive exterior body panel applications.
Of these materials, the reinforced reaction injection moldable material (RRIM) is of particular interest to the automotive industry because it can be processed economically with relatively low cost tooling. RRIM materials filled with glass fiber mats have greatly improved stiffness and dimensional stability and therefore are especially suitable for many automotive exterior body panel applications. The processing of such glass fiber mat reinforced RIM is relatively simple. The process involves placing a glass fiber mat in a mold cavity and injecting RIM material into the closed mold such that the RIM material is soaked through the glass fiber mat. A completed plastic part can be removed after it is cured in the mold.
A typical RIM material used in this process is a polyurethane based material produced from two components, an isocyanate and a polyol. The in-mold pressure of such a material system used in a RIM process is typically less than 50 psi. As a consequence, low cost tooling having low clamping force can be used for the process.
The dimensional stability of glass fiber mat reinforced RIM material is greatly improved over that of a RIM material reinforced with milled glass. As a matter of fact, the dimensional stability of glass fiber mat reinforced RIM is even superior to that of aluminum. This superior dimensional stability is achieved while other traditional benefits of RRIM, i.e., stiffness, strength, and ease of processing are maintained.
In the processing of glass fiber mat reinforced RIM materials, a new problem was discovered. This is generally called a fiber readout defect observed in the surface layer of a glass fiber mat reinforced RIM part. It is a particularly serious problem when panels are used for exterior automotive body applications where aesthetic quality is of prime importance.
The fiber readout problem is caused largely by the presence of glass fiber in the surface layer of a plastic panel. When a panel is situated in a mold under compression, resin material located between the panel surface and a glass fiber in the surface layer of the panel is under higher pressure than that located not adjacent to a glass fiber. As a consequence, when the part is demolded, the cured material located adjacent to a glass fiber will expand more than the resin material not adjacent to a glass fiber. This results in a panel having a surface showing protruding contours of glass fibers which are located immediately below the surface of the panel, commonly known as the fiber readout phenomenon.
Numerous efforts have been made to correct the fiber readout defect observed in glass fiber mat reinforced plastic panels. These efforts include the inventor's previous U.S. patent application Ser. No. 669,824, filed Nov. 9, 1984, now U.S. Pat. No. 4,610,835 issued Sept. 9, 1986, and assigned to the assignee of this invention. In that application, the inventor has shown that the application of a top coating layer in a secondary molding process could conceal fiber readout if pressure was released before the end of the molding cycle. However, this process requires delicate process control which may not be feasible in a mass production process.
It is therefore an object of the present invention to provide a method of producing glass fiber mat reinforced RIM panels without the fiber readout defect which is readily adaptable to a mass production process.
It is another object of the present invention to provide a method of making glass fiber mat reinforced RIM panels where glass fibers are kept away at a distance from the surface in a precisely controlled manner to avoid fiber readout defect.
It is yet another object of the present invention to provide a method of making glass fiber mat reinforced RIM panels without the fiber readout defect such that panels can be readily decorated to produce a predecorated surface for automotive exterior body panel applications.